1. The Field of the Invention
This invention relates to equipment for maintaining and repairing heat exchangers and, more particularly, to novel systems and methods for use in pulling tubes from heat exchangers for replacement.
2. The Background Art
A heat exchanger is any apparatus constructed for transferring heat from one object or medium to another. One type of common heat exchanger passes one fluid through tubes, and a second fluid flows over the outside of the tubes. Heat is exchanged between the fluid inside the tube and the fluid outside the tube. Heat will flow from the hotter fluid to the cooler fluid. Tubes may be provided with fins on an inside surface or on an outside surface to improve performance. Either fluid (inside or outside the tube) may be a liquid or a gas. A most common example of a heat exchanger is an automobile radiator. Another is a cooling coil on the back of a household refrigerator. Less visible, but equally common, heat exchangers include water heater cores, home furnace boilers, industrial boilers, and expansion coils in an air conditioner.
Industrial heat exchangers are often of the "shell-and-tube" type. A long shell or tank is provided with a bulkhead near each end. The bulkheads are provided with a latticework of closely spaced circular perforations. The perforations are sized to receive long tubes. Tubes fitted into the perforations extend from one bulkhead to the other. Tubes may be welded or swaged into place in the bulkheads, each forming a fluid-tight seal.
A shell-and-tube heat exchanger is typically assembled with one end of the tank and one bulkhead forming a first chamber or plenum. This end of the tank is provided with an inlet line (inlet pipe) for receiving fluid. The opposite end of the tank and the other bulkhead form a second plenum having an exit line (outlet pipe) for discharging fluid from the heat exchanger. Fluid can pass through the inlet line to the first plenum, into the tubes passing through the perforations in the first bulkhead, through the length of the tubes, out of the tubes at their ends passing through the second bulkhead, into the second plenum, and out of the second plenum through the exit line.
The bulkheads form another chamber or plenum. The walls of this plenum are formed by the bulkheads, the shell or tank wall extending between the bulkheads, and the outer surface of the tubes. An inlet line and outlet line pass fluid into and out of this plenum. Within the plenum, the fluid passes over the outside surfaces of the tubes "bundled" close to one another by the bulkheads. Thus, heat is transferred between the fluid inside the tubes and the fluid outside the tubes.
When tubes have been used for their useful life, they may be corroded (rust for example) or fouled (covered or plugged up by deposits). Corrosion may thin the walls of the tubes or pit them. Fouling typically occurs as stone-like deposits of various compounds precipitate out of the fluids passing through the tubes. Fouling deposits may accumulate on the inside surface, outside surface, or both, of a tube. Deposits inside a tube may completely block the tube, creating a rock-like "plug" in the tube.
Some heat exchangers have tubes that can be replaced. The shell is opened, exposing the bulkheads with their banks of tubes. Each tube is first broken free from the bulkhead (the "breaking" operation). Breaking often employs a combination of cutting or pressing, followed by a pull of a few inches. A hydraulic press of the collet type or the screw type may typically be used. The press engages one end of the tube and draws the tube through the bulkhead a distance of several inches.
After the tube is broken free, it must be removed. The hydraulic press cannot remove the tube. A longer stroke than that of the hydraulic press is required. Therefore, a mechanism is required to grasp a tube and draw it out quickly and completely from the bulkhead.
The hydraulic press has a very high force (tens of tons) over a very short stroke (distance of several inches). Compared to the hydraulic press, a lesser force (less than a ton, often less than 100 pounds) is adequate for removal of a tube. However the tube must be moved its entire length. The length of a tube may be from several feet to several dozen feet.
Numerous patents and other technical articles disclose methods for removing long tubes. The pullers for long tubes are sometimes referred to as travelers. The term "traveler" emphasizes the nature of the longer pulling operation at reduced force (the "traveling" operation) as compared to the short, powerful stroke of the hydraulic press in the "breaking" operation.
A traveler typically includes multiple, powered, synchronized wheels rotating opposite one another in close proximity. These "drive wheels" grasp and crush a tube between them. Helical springs provide the crushing force to keep the drive wheels close together. As a tube is crushed, it passes between the powered rollers.
The machinery for traveling is usually large and heavy. The operators must move the heavy traveler into position. Because the traveler is large, operators may not be able to see around it. Operators may have difficulty guiding the traveler toward the end of the tube if they cannot see it.
Also, the tube is ejected out the back of the traveler very rapidly. Moreover, the tube may be free to warp in any direction, because it is crushed, sometimes to a very thin, ribbon-like appearance. Unless one operator holds onto the tube to guide it, the tube may injure people, damage machinery, or clutter the work area.
The size also interferes with the breaking operation. If more than one tube extends away from the bulkhead (also called a tube sheet) within an area the size of the front face of the traveler, the traveler cannot reach the tubes. Each tube interferes with the traveler's approach to the other tubes. Thus, only one tube within a large area can be broken free in the breaking operation before the traveling operation must occur. That is, one tube is typically broken free, after which that tube must be traveled before the next tube can be broken free.
The hydraulic press is operated by a "breaking" operator, and the traveler is operated by a "traveling" operator. The breaking operator and the traveling operator must wait for each other, effectively operating in series. Only one operator and one machine are active at any time. Thus, one operator and one machine are idled (wasted) at all times.
The frequent exchanging of positions between the breaking and traveling operators adds additional wasted time. Moreover, the traveler must be moved large distances to accommodate the frequent exchange of positions. Safety is a concern in this circumstance where large pieces of heavy, powered equipment are moved rapidly about a constricted working area by a team of at least three workers (for breaking, traveling, and tube guiding at the exit of the traveler, respectively).
The great weight of the traveler makes large hoists necessary to move the traveler between tubes in a single bundle. A hoist requires additional controls to move the traveler up, down, right, left, forward and backward to the tube. These controls must be operated in series with the traveling operation, distracting the operator. Of course, the positioning of the traveler with the hoist is idle time in which tubes are not being traveled.
The size of the traveler is largely due to the complex machinery, multiple hoses, multiple hydraulic motors, and mounting hardware required to support the hydraulic loads.
In addition, the reliability of a machine is dependent on the number of parts, particularly moving parts. The travelers typically used have numerous, powered, moving parts.
Another difficulty with current travelers is their reliance on multiple drive wheels for engaging the tube. Making one drive wheel movable with respect to another adds parts, cost, complexity, weight and size. However relative translation is necessary for accommodating "plugs," large blockages of fouling deposits inside a tube.
If a plug is encountered in the tube, the traveler may be damaged. Some travelers release the drive wheels drawing the tube. However, the wheels typically can move only a very short distance apart. Moreover, the helical spring forces are not balanced or proportional with respect to the driving forces exerted by the drive wheels. Thus, the tube may stall or not engage properly when the drive wheels spin against the tube. If springs holding the drive wheels together are too strong, the drive wheels may not open properly at their nip point to grasp a larger tube and crush it initially. Yet, if springs are not strong enough, crushing may be hampered.
The size of the nip between drive wheels must be large enough to feed a tube, yet small enough to permit grasping the tube after crushing. Too high a spring load makes engagement of the tube difficult in the nip. Too low a spring load hampers crushing and may allow slipping when a plug is encountered. The crushing forces are not typically balanced with the driving forces of the drive motors.
Travelers are not typically guarded. A crushed tube is simply ejected from the back side of the traveler. The tube may bend in any direction, striking workers. A second person, in addition to the traveling operator is typically required to grasp a long, exiting tube and direct it.